Modern UV-lithography is searching for new highly parallel writing concepts. Spatial light modulation (SLM) with optical MEMS devices offers such possibilities. An SLM chip may comprise a DRAM-like CMOS circuitry with several million individually addressable pixels on top. Said pixels are deflected due to a difference in electrostatic force between a mirror element and an address electrode. A pattern generator using the SLM is described in U.S. Pat. No. 6,373,619 assigned to the same assignee as this invention. This patent discloses in short a small field stepper, which exposes a series of images of the SLM. A workpiece is arranged on a stage, which is continuously moving and a pulsed electromagnetic radiation source (which could be a pulsed laser, a flash lamp, a flash from a synchrotron light source, etc) flashes and freezes an image of the SLM on the workpiece. The SLM is reprogrammed with a new pattern before each flash so a contiguous image is composed on the workpiece.
The SLM with an array of actuators used in a mask-writing tool is loaded with a specific pattern, where each actuator is in an addressed or a non-addressed state before each stamp is printed. This pattern is a subset of the pattern to be printed on the mask. The SLM is loaded with patterns in this way a number of times for each strip. After the writing of the strip consisting of a series of stamps over the full length of a mask substrate, the stage is moved back to an initial position in preparation for writing of the next strip.
Charging of micro mirror surfaces or addressing electrode surfaces, hereafter called “charging”, is a cumulative effect that may limit the performance of micro mechanical actuators. The charging effect manifests itself by a gradually increased change of the actuator position when all parameters are kept constant in a deflected state. It further manifests itself by curing out when the actuator is left in the non-addressed state for a sufficient amount of time. Both the build-up and curing-out are non-linear effects. The resulting position at a given time thus depends not only on the addressing voltage at that time, but also on the history of addressing of this particular actuator element. Therefore the response of the actuator may become inaccurate resulting in a reduced placement accuracy of features on a workpiece in the lithography process, which is a problem.